With the recent rapid progress of portable electronic equipment and communication equipment, secondary batteries having a high energy density are strongly desired from the standpoints of economy and size and weight reduction. Prior art known attempts for increasing the capacity of such secondary batteries include the use as the negative electrode material of oxides of V, Si, B, Zr, Sn or the like or compound oxides thereof (for example, Patent Document 1: JP-A H05-174818, Patent Document 2: JP-A H06-60867), melt quenched metal oxides (for example, Patent Document 3: JP-A H10-294112), silicon oxide (for example, Patent Document 4: JP 2997741), and Si2N2O or Ge2N2O (for example, Patent Document 5: JP-A H11-102705).
These prior art methods are successful in increasing the charge/discharge capacity and the energy density of secondary batteries, but fall short of the market demand partly because of insufficient cycle performance and are still unsatisfactory. A further improvement in energy density is desired.
More particularly, Patent Document 4: JP 2997741 describes a high capacity electrode using silicon oxide as the negative electrode material in a lithium ion secondary cell. Because of an increased irreversible capacity on the first charge/discharge cycle and a practically unacceptable level of cycle performance, there is still left room for improvement. Thus Patent Document 6: JP 3952180 and Patent Document 7: JP 4081676 propose improvements in first cycle efficiency and cycle performance. On the other hand, when secondary batteries are used in electric vehicles, the key factor is a high energy efficiency. Specifically, the important task is an improvement in coulomb efficiency (discharge electric quantity/charge electric quantity) which is a ratio of the quantity of electricity removed from a battery during discharge (discharge electric quantity) to the quantity of electricity needed during charging (charge electric quantity).